The present invention relates generally to ignition and control systems for operating a burner, and, more particularly, to dual solenoid burner control systems for gas clothes dryers.
A variety of ignition and control systems for gas clothes dryers have been proposed, all having as a general objective the provision of a relatively low-cost system which, at the same time, is reliable and effective. It will be appreciated that reliability is an important consideration, particularly where a gas appliance is involved.
In the interest of surety of operation, it is usual practice to provide such systems with a pair of electromagnetically operated solenoid gas valves arranged in series with the burner gas supply conduit such that fuel can be supplied to the burner only when both valves are open.
Two other usual elements in such systems are an igniter element, such as a resistance glow-type igniter which, when energized, heats to a temperature sufficiently high to ignite gas; and a thermal sensing switch having contacts which are closed when cold and open when hot. Advantageously, the sensing switch is typically positioned so as to be responsive both to the heat from the igniter and to the heat from the burner flame, when present. For low cost, it is desirable to be able to use a thermal sensing switch having single pole contacts.
An additional element of typical prior art systems is a holding or latching circuit arrangement for one of the gas valves arranged such that, during normal operation when a flame has been established, the one gas valve is held in an energized condition only by means of its latching or holding circuit. Even a momentary interruption in electrical power supplied to the circuit causes this valve to close, shutting off of the gas supply to the burner. Thereafter, a proper reignition sequence is required to get gas flow and flame.
In the normal operation of such systems, initially the thermal sensing switch is cold and its contacts are closed. When power is supplied to the system, the igniter and a first gas valve are energized. However, the second gas valve is not energized at this point, and for an initial interval no gas enters the combustion chamber. Typically, during this interval a blower operates and purges the combustion chamber of any unburned fuel which may be remaining from a previous operation. When the igniter exceeds ignition temperature, the thermal switch contacts open. Opening of the thermal switch contacts energizes the second gas valve so that gas is free to enter the combustion chamber and to be ignited. At the same time, the igniter is either fully or partially de-energized, depending upon the particular system, and cools down. However, the thermal mass of the igniter allows it to remain above ignition temperature long enough to ignite the entering gas. After normal ignition, flame heat keeps the contacts of the thermal sensing switch open, even though the igniter itself is no longer fully energized.
These various prior art circuits also deal in various manners with abnormal conditions. One such abnormal condition, already mentioned, is a momentary interruption in electrical power supplied to the system, such as would result with a momentary dryer door opening. When this occurs, the latching or holding circuit for one of the gas valves is broken, shutting off the supply of gas to the burner, and requiring a proper reignition sequence with a purging operation.
Another abnormal condition is failure of the gas to ignite, even though the igniter has reached an ignition condition. In typical circuits, when ignition fails to occur, the thermal sensing switch cools down, and its contacts assume their normal, closed position, causing at least one of the gas valves to close, shutting off the supply of gas to the burner. At this point, the system may recycle for another and subsequent restart attempts, until such time as ignition occurs or another means terminates operation of the system.
By way of example, the following U.S. Patents are identified as disclosing examples of various systems of the general type described above, particularly adapted for gas clothes dryers: Place et al U.S. Pat. Nos. 3,497,849 and 3,589,846; Elders U.S. Pat. No. 3,597,139; Good U.S. Pat. No. 3,603,708; Fox U.S. Pat. No. 3,620,659; Cahoe et al. U.S. Pat. No. 3,806,308; Fernstrom U.S. Pat. No. 3,807,933; and Hantack U.S. Pat. No. 3,862,820. These patents, although all generally similar in function and in components employed, describe a number and variety of circuits for providing the various functional requirements.
These patents also illustrate a number of design principles which have been applied to such circuits. For the holding or latching function mentioned above, at least three approaches have been employed.
A first approach involves providing at least two windings on one of the solenoid gas valves, one of the windings being a holding winding which may be connected directly across the incoming power lines. The holding winding itself is insufficient to actually operate the valve, for which the second winding is provided. As is known, solenoid operated gas valves normally require much more magnetic force to initially operate than to hold in an operated condition. Examples of this first approach are provided by the disclosures of the Elders U.S. Pat. No. 3,597,139 and the Cahoe et al U.S. Pat. No. 3,806,308.
A second approach to providing holding or latching circuit does not require a dual coil gas valve solenoid, and instead relies upon a holding current resistor connected in series with a gas valve winding. The holding current resistor is selected such that current passed therethrough is insufficient to initially cause operation of the gas valve, but sufficient to keep the valve operated once it is operated. A switching means is provided to effectively by-pass the holding current resistor for initially operating the valve. An example of this second type of holding circuit is provided by the disclosure of the Fox U.S. Pat. No. 3,620,659. Further, although not a dual solenoid valve gas dryer control circuit, the disclosure of the Hirschbrunner U.S. Pat. No. 3,600,117 discloses a similar resistor holding circuit for a solenoid fuel valve, wherein a triac bypasses the resistor to initially operate the valve.
A third approach to providing holding or latching circuit employs special relay or switching contacts mechanically connected to and operated by one of the solenoid valves. The electrical connection is such that a circuit to the valve winding is completed through the special contacts. Such a circuit is disclosed in the Place et al U.S. Pat. Nos. 3,497,849 and 3,589,846.
As an example of another usual design principle, for initially energizing the igniter and positively precluding operation of one of the gas valves while the thermal sensing switch contacts are closed, it is typical to electrically connect the igniter and the first valve winding in series between the power input conductors, and to effectively connect the thermal sensing switch contacts in shunt across the second valve winding. With this arrangement, when the thermal sensing switch contacts later open, the igniter is de-energized, and the second solenoid winding is energized.
The present invention provides a system of the general type described above which provides has all of the usually required operational functions, and yet is extremely uncomplicated and low in cost. Further, the present invention allows the use of standard components, such as single winding gas valves and a single pole thermal sensing switch of conventional design. The present invention, in its preferred embodiment, provides the additional function of de-energizing the first solenoid valve should the second solenoid valve fail to operate due to a mechanical malfunction even though the second solenoid valve is energized.